Usually around the same point in a society’s development it was understood that - unlike the planet where the two-dimensional creatures lived - space was not simply curved into a hypersphere, it was also expanding; gradually increasing in size like a soap-bubble on the end of a straw which somebody was blowing into. To a four-dimensional being looking from far enough away, the three-dimensional galaxies would look like tiny designs imprinted onto the surface of that expanding bubble, each of them, generally, heading away from all the others because of the hypersphere’s general expansion, but - like the shifting whorls and loops of colour visible on the skin of a soap bubble - able to slide and move around on that surface.

Of course, the four-dimensional hypersphere had no equivalent of the straw, blowing air in from outside. The hypersphere was expanding all by itself, like a four-dimensional explosion, with the implication that, once, it had been simply a point; a tiny seed which had indeed exploded. That detonation had created - or at least had produced - matter and energy, time and the physical laws themselves. Later - cooling, coalescing and changing over immense amounts of time and expansion - it had given rise to the cool, ordered, three-dimensional universe which people could see around them.

Eventually in the progress of a technologically advanced society, occasionally after some sort of limited access to hyperspace, more usually after theoretical work, it was realised that the soap bubble was not alone. The expanding universe lay inside a larger one, which in turn was entirely enclosed by a bubble of space-time with a still greater diameter. The same applied within the universe you happened to find yourself on/in; there were smaller, younger universes inside it, nested within like layers of paper round a much-wrapped spherical present.

In the very centre of all the concentric, inflating universes lay the place they had each originated from, where every now and again a cosmic fireball blinked into existence, detonating once more to produce another universe, its successive outpourings of creation like the explosions of some vast combustion engine, and the universes its pulsing exhaust.

There was more; complications in seven dimensions and beyond that involved a giant torus on which the 3-D universe could be described as a circle, contained and containing other nested tori, with further implications of whole populations of such meta-Realities . . . but the implications of multiple, concentric, sequential universes was generally considered enough to be going on with for the moment.